Researchers are hoping this world of kindly dads and intolerant moms, of males who are merely attentive and others who've become downright clingy, can shed light on the biology underlying human behavior.

"This is about as close as we can get to being related to us in an animal that's monogamous" and that can be studied easily in a lab, said Karen Bales, a UC Davis psychology professor.

The titi monkeys housed at the California National Primate Research Center comprise the nation's only research colony of a South American species well-known for its steadfast monogamy, along with a rough brand of mothering that could come right out of a tabloid tell-all.

"In most monogamous species, the dads are really good. It's just taken to an extreme in titi monkeys because the moms are kind of bad," Bales said. "Sometimes you watch a titi monkey mom and you feel like she doesn't like her babies."

Titi babies tend to ride draped across a parent's shoulders, and when mom wants the kid off her back, her favorite strategy for shifting responsibility is to make the baby cry.

"She'll rub it up against the side of the cage, or in the wild against a tree branch, to make it cry, or nip it a little, and then daddy will come get it," Bales said.

Both parents will come running to their baby's cry if researchers place the infant on the ground, but mom will often pick it up and hand it to dad.

There are exceptions, and the primate center's 64-titi colony houses one unusually doting mother.

It's also got a smattering of kids who spend lots of time toting younger siblings, and Bates and Sally Mendoza, a UC Davis psychology professor who focuses on monogamy, have a theory about why.

Mendoza, who's been studying titi monkeys for decades, was trying to create a handful of standoffish titi males by suctioning out a small section of their brains that seemed to regulate social behavior.

She thinks she may have disrupted a part of the brain that regulates addictive behavior, basically making the monkeys unusually addicted to their sweethearts.

As dads, the surgically altered males seem especially tolerant of their kids, which in turn could be creating more hands-on big brothers and sisters.

"It could be just a chance thing. We don't know yet," Mendoza said. But the fathers are raising "a group of kids which are carrying their siblings as much as 25 percent of the time, which is unusual."

Now, Mendoza and Bates want to look at the long-term impact of these superdads on future generations. Will their offspring be more attentive fathers?

The researchers have applied for a grant to compare the children of these titi monkeys with those of "bad" dads, the ones who groom their youngsters less, carry them less often and have a lower offspring survival rate.

In time, they'd like to know not only what creates the healthiest kids, but also whether there are ways to undo the damage inflicted by poor parenting.

Ultimately, Mendoza believes the neurobiology of monkey relationships - what's happening in the brain when boy meets girl or when a baby arrives - will provide a key to understanding human health.

"We know that sociality has a big impact on every disease process," she said. "Just about any disease you can imagine is probably made worse by not having a completely functional, supportive social environment."

But we don't know what exactly happens in our minds and in our bodies to confer extra protection to those with strong social circles.

Among the puzzles, said Mendoza: How does a love relationship stack up against friendships in bolstering health and disease resistance?

In her efforts to better understand pair bonding, she and Bates have also been looking at the titi monkey equivalent of young love. They've been doing brain imaging studies comparing long-established couples with pairs matched by researchers after their first 48 hours together.

The preliminary results might not surprise anyone who's ever been in the throes of new love.

"These are different states of being," said Bales, with the newly matched undergoing a storm of changes in "multiple, multiple brain areas" after boy meets girl.

There's much greater activity in parts of the brain that regulate social recognition, anxiety, aggression, the reward-pleasure system, and, of course, sex.